The invention relates to a system for transferring data and to the use of this system, especially to setting images on thermal printing plates.
In printing technology, especially in offset printing technology, a distinction is in principle drawn between the analogue and the digital setting of printing plate images:
The data contained on a negative or positive film, such as images, text or graphics, are transferred to a light-sensitive printing plate--in an analogue manner--in a copying room or a repetition copying machine by means of an ultraviolet light source. The property of the light-sensitive layer on the printing plate is matched to the light source, so that it has a sensitivity maximum at 380 nm, for example.
The image area of a printing plate is written directly by one or more lasers--in a digital manner. The sensitivity of the printing plate has its maximum corresponding to the light source. Depending on the wavelength of the light used, one speaks of laser plates in the case of visible light and of thermal plates in the case of infrared light at, for example, 830 nm.
Nowadays, thermal plates have a particularly promising future. These pre-coated, anodized aluminum plates can have images set both by analogue and by digital means. The maximum sensitivity of the plates, for example at 830 nm, is selected such that normal daylight does not have a disruptive effect t during "image-setting". Above this value, the sensitivity of the thermal plates drops off abruptly, so that, for example at a wavelength of 880 nm, only about 10-15% of the sensitivity measured at 800 nm is still achieved. Experience shows that an energy requirement of 170 mJ/cm.sup.2 is needed to set an image on such thermal plates. The use of thermal plates renders superfluous complicated and therefore expensive handling systems which permit light-sensitive plates to have images set under dark-room conditions. In spite of their ability to be handled simply, thermal plates, thanks to the infrared or thermal technology which can be used, ensure high reliability. In the meantime, there are thermal plates on the market which, on the one hand, permit conventional positive processing with aqueous development and which, on the other hand, can be used in the printing machine like conventional coated printing plates. In addition, with the increasing digitization of the printing trade, the requirement for the digital image-setting of printing plates is growing.
In order to transfer data, such as images, texts or graphics, to data carriers, such as offset printing plates, essentially three image-setting principles are known.
The flat bed, as the simplest system for the analogue image-setting of printing plates, is suitable only to a certain extent for the digital image-setting of the same. On the one hand, the outlay for guiding an image-setting head increases with the size of the printing plate, on the other hand there is always a part of the possible travel of the image-setting system which is unused, as a result of the demanded image-setting speed and because of the necessary starting and braking ramps of the image-setting head.
An internal-drum image setter (see FIG. 1) which is known per se has the following advantages: the plate size can be selected independently, since the plate does not move during image-setting. The plates can be simply laid in fitting holes provided. An automatic system for leading the plates to and from can be provided relatively simply. The disadvantages of such an internal-drum image setter have to be listed: it is necessary to operate with a particularly powerful, frequency-doubled YAG laser, a single laser beam being caused to rotate over the plate from the centre of the drum. This makes focusing more difficult, because of the great distance, and does not permit image-setting using multiple beans. Use is often made of a polygon mirror, which is rotated at up to 30,000 rev/min. This requires a complicated mounting. In addition, vibrations--be they in the environment of the device or within the system--have an extreme effect on the achievable resolution.
In an external-drum image setter (see FIG. 2) which is likewise known, the image-setting head can be brought up to a few millimetres from the plate, which permits the use of significantly weaker and cheaper laser diodes. The latter can be more easily focused and added as multiple heads, devices having alternatively 256 or 512 laser diodes arranged alongside one another being known. These advantages are opposed by the following disadvantages: secure fixing of the plate on the drum is very complicated in design terms. Undesired detachment of the plate during the image-setting cannot be ruled out. However, should this occur, considerable damage to the machine, especially to the image-setting head, must be expected. The rotational speed of the drum is limited to about 100 rev/min, which means that an enlargement of the area on which the image is to be set affects the image-setting time to a considerable extent. Various plate formats result in a different distribution of weight on the drum, which, in routine operation, results in the necessity for automatic balancing. Inserting the plates securely into the register pins is very complicated, which virtually requires the automation of this process, on the one hand, but makes it considerably more difficult, on the other hand.
EP 0 590 907 discloses a rotating drum for a scanner or plotter. This is an internal-drum image setter, in which the drum rotates rather than the light source. The light source is fixed to a long arm, which reaches over the entire length of the drum and penetrates into the latter, so that image-setting over the entire area of substrates fixed to the inside of the drum is made possible. The long, linearly movable arm can pick up oscillations in the system and, as a result, begin to oscillate itself. Such a behavior of the arm can have a detrimental effect on the resolution of the image.
U.S. Pat. No. 4,479,133 discloses a light beam rotary printer having a light beam projection system which comprises a number of light sources each having a condenser lens. These light sources are fixed to a rotating wheel and radiate outwards in the radial direction while the wheel is rotated. In order to be able to produce uniform image-setting with this internal-drum image setter, the individual light sources have to be adjusted finely in the range of 12 .mu.m. This extremely delicate setting operation means additional working outlay and necessitates the use of specially trained personnel; both can have the effect of making the products more expensive or--if too little attention is paid to this point--can have a detrimental effect on the resolution of the image.
The object of the present invention is to propose an image-setting system which, on the one hand, combines many of the individual advantages of the known internal-drum and external-drum image setters and, in addition, eliminates a considerable number of the disadvantages of these systems.